Physiological processes invariably depend on specific interactions between biological binding partners. Since receptors, transport proteins, enzymes, and antibodies are able to differentiate between the stereoisomers of chiral compounds, one stereoisomer is generally preferred in, e.g., metabolic pathways and cellular communication. Thus, living organisms discriminate between the enantiomers of endogenous and exogenous compounds, such as drugs, at virtually all levels of interaction, and respond differently to them. While one enantiomer of a drug may exhibit a desired activity, the other may cause severe pharmacologic and toxicologic side effects, or act as an antagonist. Despite the fact that the stereoselectivity of protein-ligand interactions was first recognized more than a century ago, the molecular basis of such chiral discrimination is not yet understood. Our long-term objective is to elucidate the structural elements that govern chiral discrimination at the molecular level by studying the stereoselective interaction between antibodies and low-molecular weight chiral compounds as a model system. The antigen binding fragments of several antibodies that stereoselectively recognize the D- or L-enantiomers of amino acids or hydroxy acids will be crystallized for structural analysis by X-ray crystallography. While the comparison of the structures of, e.g., anti-D-amino acid antibodies and anti-L-amino acid antibodies in complex with D- and L-amino acids, respectively, will reveal, which residues in the antibody binding site mediate stereoselective interaction with the ligand, the assessment of individual anti-amino acid antibodies with and without ligand will allow us to evaluate the importance of conformational changes for stereoselective binding. Analogous studies with the anti-hydroxy acid antibodies will provide additional information about the effect of substituting functional groups about the stereogenic center. Utilizing the class-specificity of the antibodies under study, structures of selected antibodies in complex with different ligands will disclose the contribution of particular residues to stereoselectivity and overall binding activity. The production of single chain variable fragments and their structure-guided manipulation by site-directed mutagenesis will be used to test the validity of our findings and to create antibodies with tailor-made binding properties. Success in this project will provide new, unprecedented insights into the molecular basis of biorecognition and impact upon both, basic and applied science, and thus human health. A better comprehension of the molecular basis of protein stereoselectivity will, for example, be valuable for the rational design of more efficient drugs and the development of tailor-made chiral host molecules for the analysis, separation, and synthesis of stereoisomeric compounds. [unreadable] [unreadable] [unreadable]